The present invention relates generally to impact printing devices for dot matrix printers and, more particularly, to improved solenoid-type actuators for such printing devices.
A typical print head for a dot matrix-type printer has a plurality of print wires, each actuated by an individual print wire solenoid. This type of print wire solenoid actuator is disclosed, for example, in U.S. Pat. No. 3,755,700, issued Aug. 28, 1973. The solenoid actuator consists of a magnetically permeable cylinder which serves as a bobbin on which the electromagnetic coil is wound, as a guide for an armature which is disposed within the cylinder, and as part of the path for the magnetic flux. The cylinder is enclosed within a housing structure of magnetically permeable material which also forms a part of the magnetic circuit. The cylinder has an annular gap therein intermediate its ends, which may be formed by inclusion of a section of non-magnetic material, and which interrupts the magnetic circuit formed by the cylinder and the outer structure. The armature serves to complete the magnetic circuit past this gap.
The armature, which is connected to the print wire, moves axially of the cylinder between a normal rest position and a printing position. In its rest position, the armature, in the form of a piston, has its leading edge disposed at the cylinder gap and is so related thereto that there results a relatively long magnetic flux path at the leading edge of the armature. Thus, when the coil is energized, the axial component of the magnetic force exerted on the piston or armature moves it forwardly to its printing position which is toward an equilibrium position wherein the armature extends substantially equal distances forwardly and rearwardly of the cylinder gap so that the axial components of the magnetic force exerted thereon in the forward and rearward directions substantially balance each other and could be essentially zero. When the coil is de-energized, the armature is returned to its rest position (i.e. seated against a back step) under the action of a bias spring and the rebounding of the piston and wire mass from the target (i.e. the platen), ribbon and record medium.
Because the print head includes a plurality of print wires, each controlled by its own solenoid, and because the print wires must be closely grouped to print relatively small characters, it is essential that the size of the solenoid-type print wire actuators be minimized. Since the armature is the innermost component of the actuator assembly, the overall outer diameter of the actuator will be a function of the armature diameter. Thus, it is desirable to make the armature diameter as small as possible. However, it has been found that when the diameter of the armature of prior actuators is reduced, it results in a corresponding reduction in the drive force imparted to the print wire. The reduced drive force results in a reduced armature acceleration and reduced impact force when the wire hits the target. This reduced impact force is unacceptable, since it is not sufficient for clear printing.
Furthermore, the prior actuator solenoid is encased in a magnetic cylinder, the thickness of which serves to add to the overall size of the device. This serves further to increase the center-to-center distance of the print wires when the actuators are arranged in a linear array, or the size of the cluster when arranged in a circular or other non-linear array.
Additionally, the formation of the gap in the cylinder results in a multi-part construction of the cylinder, necessitating costly and complex manufacture.